1. Introduction
This invention relates to electroplating nonconductors in a selective pattern. More particularly, the invention relates to an additive electroplating process for depositing metal in a desired pattern using an electrically conductive, non-metallic conversion coating. The invention is especially useful for an additive or semi-additive process for the manufacture of printed circuit boards where copper is deposited over a circuit board base material coated with a metal chalcogenide conversion coating.
2. Description of the Prior Art
Nonconductive surfaces are conventionally metalized by a sequence of steps comprising catalysis of the surface of the nonconductor to render the same catalytic to an electroless metal deposit followed by contact of the catalyzed surface with an electroless plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Metal plating continues for a time sufficient to form a metal deposit of the desired thickness. Following electroless metal deposition, the electroless metal deposit is optionally enhanced by electrodeposition of a metal over the electroless metal coating to a desired full thickness. Electrolytic deposition is possible because the electroless metal deposit serves as a conductive coating that permits electroplating.
Catalyst compositions used for electroless metal plating are known in the art and disclosed in numerous publications including U.S. Pat. No. 3,011,920, incorporated herein by reference. The catalysts of this patent consist of an aqueous suspension of a tin noble or precious (catalytic) metal colloid. Surfaces treated with such catalysts promote the generation of electrolessly formed metal deposits by the oxidation of a reducing agent in an electroless plating solution catalyzed by the catalytic colloid.
Electroless plating solutions are aqueous solutions containing dissolved metal and reducing agents in solution. The presence of dissolved metal and reducing agent together in solution can also result in solution instability and indiscriminate deposition of metal on the walls of containers for such plating solutions. This may necessitate interruption of the plating operation, removal of the plating solution from the tank and cleaning of tank walls and bottoms by means of an etching operation. Indiscriminate deposition may be avoided by careful control of the plating solution during use and by the use of stabilizers which inhibit indiscriminate deposition, but which also retard plating rate and often adversely effect deposit quality.
Attempts have been made in the past to avoid the need for an electroless plating solution by a direct plating process whereby a metal may be deposited directly over a treated nonconductive surface. One such process is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process disclosed in this patent involves treatment of the nonconductive surface with a tin palladium colloid which forms an essentially nonconductive film of colloidal palladium particles over the nonconductive surface. This may be the same tin palladium colloid used as a plating catalyst for electroless metal deposition. For reasons not fully understood, it is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation from a conductive surface. Therefore, deposition begins at the interface of a conductive surface and the catalyzed nonconductive surface. The deposit grows laterally along the catalyzed surface from this interface. For this reason, metal deposition onto the substrate using this process is slow. Moreover, deposit thickness is uneven with the thickest deposit occurring at the interface with the conductive surface and the thinnest deposit occurring at a point most remote from said interface.
An improvement in the process of U.S. Pat. No. 3,099,608 is disclosed in U.K. Patent No. 2,123,036B, incorporated herein by reference. In accordance with the process described in this patent, a surface is provided with metallic sites and the surface is then electroplated from an electroplating solution containing an additive that is said to inhibit deposition of metal on the metal surface formed by plating without inhibiting deposition on the metallic Sites over the nonconductive surface. In this way, there is said to be preferential deposition over the metallic sites with a concomitant increase in the overall plating rate and deposit uniformity. In accordance with the patent, the metallic sites are preferably formed in the same manner as in the aforesaid U.S. Pat. No. 3,099,608--i.e., by immersion of the nonconductive surface in a solution of a tin palladium colloid. The additive in the electroplating solution responsible for inhibiting deposition is described as one selected from the group of dyes, surfactants, chelating agents, brighteners and leveling agents. Many of such materials are conventional additives for electroplating solutions.
There are limitations to the above process. Both the processes of the U.S. and the U.K. patents for direct electroplating require conductive surfaces for initiation and propagation of the electroplated metal deposit. For this reason, the processes are limited in their application to metal plating of nonconductive substrates in areas in close proximity to a conductive surface.
U.S. Pat. No. 5,017,742, incorporated herein by reference, discloses a new method for directly electroplating the surface of a conductor which is said to be an improvement over the process of the referenced U.K. patent. The invention disclosed in this patent was based upon several discoveries. One discovery was that chalcogenide films of metals that function as electroless deposition catalysts may be directly electroplated without an intermediate electrolessly deposited layer. Another discovery was that many of such films are insoluble and unaffected by treatment chemicals used for plating of plastics or for circuit board fabrication and therefore, the process of the invention was suitable for the formation of printed circuits using processes such as pattern plating with dry film photoresists.
The chalcogenide film used for direct electroplating in accordance with U.S. Pat. No. 5,017,742 is formed by a process comprising catalysis of a nonconducting substrate with an electroless plating catalyst followed by treatment of the electroless plating catalyst with a solution of a chalcogenide, preferably a solution of a sulfide or sulfide precursor, to convert the catalyst to the corresponding chalcogenide. This chalcogenide is sufficiently conductive to permit direct electroplating over its surface without the need for an intermediate electroless metal deposit. It is further known that by appropriate control of concentration limits, the entire surface of a panel may be plated without the requirement for adjacent metal contacts in close proximity to the chalcogenide film.
An additional direct plate process is disclosed in published EPO application 89/00204 published Sep. 8, 1989 and incorporated herein by reference. In this process, the surface of a substrate is pretreated with a solution having an oxidizing capability; removed from said solution and rinsed; introduced into a solution containing a monomer such as pyrrole, furane, thiophene or its derivatives, which in a polymeric or copolymeric form, is electrically conductive; the substrate is then transferred into an acidic solution whereby an electrically conductive polymer layer, such as polymerized or copolymerized pyrrol, furane, thiophene or derivatives, is formed; residual solution is removed by rinsing; and the substrate is then electroplated.
A further development in the art of electroplating of nonconductors is disclosed in U.S. Pat. No. 4,897,164 incorporated herein by reference. In this patent, the process comprises forming a liquid dispersion of carbon black, coating a substrate with the dispersion, microetching the carbon black coating to form an array of circuit lines and electroplating the remaining array. The carbon black coating is sufficiently conductive to permit electroplating to take place.